Comparison of Electrospray Mass Spectrometry of Chrysanthemic

Home , Prallethrin

RAPID COMMUNICATIONS IN MASS SPECTROMETRY, VOL. 11, 796–802 (1997) Comparison of Electrospray Mass Spectrometry of Chrysanthemic Acid Ester Pyrethroid Insecticides with Electron Ionization and Positive-ion Ammonia Chemical Ionization Methods

Ian A. Fleet1* and John J. Monaghan1,2 1Michael Barber Centre for Mass Spectrometry, Department of Chemistry, UMIST, Sackville Street, Manchester, M60 1QD, UK 2Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK

SPONSOR REFEREES: Dr K. Hunter and Mr A. Newton, Scottish Agricultural Science Agency, East Craigs, Edinburgh EH12 8NJ, UK

The synthetic chrysanthemic acid ester pyrethroid insecticides empenthrin and prallethrin were studied by positive-ion electrospray mass spectrometry (ES-MS) in the presence of ammonium acetate and formic acid. + + Ammoniated molecule base-peak ions [M + NH4] and significant protonated molecule ions [M + H] were observed at low electrospray source cone voltages for both insecticides. The effect of increasing the source cone voltage (from 10 to 40 V), in particular its influence on the extent of fragmentation arising from in-source collision-induced dissociation (CID), was also investigated and found to yield interpretable spectra. The associated increase in the population of low mass fragment ions following these CID experiments makes the monitoring of class-specific ions less attractive (poor sensitivity) than the monitoring of their respective protonated and/or ammoniated molecule ions. Key fragment ions in the ES mass spectra of both insecticides were found to be identical with those obtained under positive-ion electron ionization (EI) and positive-ion (ammonia) chemical ionization (proton transfer) conditions. Additionally, a number of these key ions have been examined by both EI tandem mass spectrometry (MS/MS) and positive-ion ES-MS/MS under low-energy collision induced dissociation (CID) conditions. © 1997 by John Wiley & Sons, Ltd. Received 28 December 1996; Accepted 10 January 1997 Rapid. Commun. Mass Spectrom. 11, 796–802 (1997) No. of Figures: 11 No. of Tables: 0 No. of Refs: 12

The synthetic pyrethroids are a structurally diverse ments (in the range 12–60 pg by selected-ion monitor- class of insecticides which have been developed following) have demonstrated the capability of ES-MS to ing continuous research efforts over the past 35 offer the analyst high sensitivity and specificity for the years.1–5 This research effort was prompted by the detection of this structurally diverse insecticide class at insecticidal properties and low mammalian toxicities of the trace level, compared with other ionization meth- the natural pyrethrins6 and a desire to produce syn- ods such as electron ionization (EI) or positive/ thetic analogues having improved photo-stability and negative-ion chemical ionization (CI) which singly may potency, although undergoing faster biodegradation not offer the desired sensitivity. and photodegradation than the more persistent chlorin- We have extended our investigations, using off-line ated pesticides. Synthetic pyrethroids are applied to positive-ion ES-MS, to the study of the synthetic crops, forests, soils and animal feeds and are in chrysanthemic acid ester pyrethroids, empenthrin and household use. The resulting loss of these compounds prallethrin (Scheme 1), Prallethrin is structurally sim- as the intact molecules, together with their degradation ilar to cinerin I, jasmolin I and pyrethrin I, which are products and/or metabolites, to the environment insecticidally active components of natural pyrethrum, requires the detection of these compounds at the and also to the synthetic pyrethroid allethrin and its microgram and sub-microgram levels. stereoisomer (bioallethrin). Empenthrin is an ethy- Electrospray ionization, which involves the desolva- nylpentenyl chrysanthemate pyrethroid. The effect of tion of charged droplets to yield free gas-phase ions increasing the ion source sampling cone voltage and its from analyte species in solution, was pioneered by Dole influence on the extent of fragmentation experienced et al.7 and, in combination with mass spectrometry,8 has by each insecticide were investigated and compared recently become attractive to the analytical chemist for with EI and positive-ion (CI) (proton transfer using its ability to ionize and detect labile, low molecular ammonia reagent gas) spectra. Selected key ions have weight compounds such as sulfonylurea herbicides.9 We been examined by EI and ES tandem mass spectrome- have previously reported the analysis of several pyr- try (MS/MS) under low-energy collision induced dis- ethroids, using on-line microbore reversed-phase high- sociation (CID) conditions. performance liquid chromatographic (HPLC) separa- tion, coupled with positive-ion electrospray mass spectrometry (ES-MS), to generate interpretable spec- EXPERIMENTAL 10 tra. Limits of detection obtained from these experi- A VG Quattro tandem quadrupole mass spectrometer (up-graded to Quattro II specifications) and MassLynx data system (VG Organic, Altrincham, Cheshire, UK) *Correspondence to: I. A. Fleet were used to carry out various positive-ion ES-MS and

ES-MS/MS experiments. All experiments were per- scan. The resulting accumulated summed scan data, for formed with the electrospray source high-voltage lens individual cone voltage experiments, were stored as held at 0.55 kV and the electrospray probe at 4.4 kV. averaged spectra. EI and positive-ion CI (ammonia) The source cone voltage was varied between 10 and 40 experiments were carried out using an electron energy V. All ES-MS experiments were carried out with the of 70 eV with the source held at 180 °C and the direct resolution set such that the peak width at half-height of insertion probe at 80 °C. the ammoniated molecule ion (empenthrin) was 0.5 u. All tandem mass spectra were obtained using argon The mass spectrometer was calibrated over the desired gas in the RF-only hexapole collision cell. The gas mass range using polyethylene glycol. pressure was adjusted so that 50% suppression of the The ﬁrst quadrupole analyser was used to study cone selected ion was obtained. The collision energy was 25 voltage induced fragmentations while CID MS/MS eV in the laboratory frame of reference for each experiments were performed on selected ions resulting experiment. from cone voltage induced fragmentation. Individual The solvents, propan-2-ol and water, were of HPLC pyrethroid standards, 25 µg mL–1, in 70:30 propan- grade (Rathburn Chemicals, Walkerburn, UK). Ammo- 2-ol + H2O containing 10 mM CH3COONH4 and 22 nium acetate, formic acid and chrysanthemum mono- –1 mM HCOOH, were infused at a rate of µL min carboxylic acid were of ACS quality (Sigma–Aldrich, through the electrospray probe, via a 700 mm ϫ 75 µm Poole, Dorset, UK). Individual 25 µg per mL–1 pyr- i.d. (375 µm o.d.) uncoated fused-silica capillary (Poly- ethroid standards were freshly prepared by serial micro Technologies, Phoenix, AZ, USA), using a 100 dilution of their respective stock standard solutions µL syringe and a syringe infusion pump (Model 22; using 70:30 propan-2-ol + H2O containing 10 mM Harvard Apparatus, South Natick, MA, USA). CH3COONH4 and 22 mM HCOOH. All standards were The positive-ion ES mass spectra of each pyrethroid, stored in 1 mL amber-glass vials at 5 °C. at cone voltages between 10 and 40 V, were acquired by The research-grade quality pyrethroid samples used scanning the mass range m/z 60–350 at a rate of 8 s per in this study were donated by Sumitomo Chemical (Osaka, Japan). Common names of each pyrethroid are used throughout this paper (see Scheme 1).

RESULTS AND DISCUSSION We studied the electrospray spectra (in the presence of ammonium acetate and formic acid) of the chrysanthemic acid ester pyrethroids empenthrin and prallethrin and compared the results with their EI and CI (ammonia) spectra.

Empenthrin (Mr = 274) At lower cone voltages (10 V), the positive-ion electrospray spectrum of empenthrin (Fig. 1) consists predom- + inantly of the ammoniated molecule, [M + NH4] (base peak, m/z 292) and a signiﬁcant protonated molecule [M + H]+ (m/z 275). Spectra recorded at progressively higher cone voltages show the effect of source-derived CID, which results in a diminution of + Scheme 1. Chrysanthemum monocarboxylic acid and pyrethroid the [M + NH4] population, with a concomitant structures. increase in the relative abundance of the [M + H]+ ion

Figure 1. Positive-ion electrospray spectrum for empenthrin (cone voltage 10 V). * Solvent ions.

© 1997 by John Wiley & Sons, Ltd. Rapid Communications in Mass Spectrometry, Vol. 11, 796–802 (1997) 798 ELECTROSPRAY MASS SPECTROMETRY OF CHRYSANTHEMIC ACID PYRETHROIDS and of diagnostically useful fragment ions (Fig. 2). The undergoes further loss, in the form of expulsion of a increased relative abundance of the protonated mole- molecule of carbon monoxide, to form the chrysanthe- cule, compared with the ammoniated molecule, at myl cation (m/z 123) is provided by a positive-ion higher cone voltages may result solely, or in part, from electrospray constant neutral-loss scan, set to monitor a + + the dissociation of [M + NH4] to [M + H] + NH3. loss of 28 u (Fig. 3 and Scheme 2, pathway 6). Confirmation that NH3 can be lost directly from the A precursor ion spectrum (not shown) of the base + [M + NH4] ion was provided by a constant neutral- peak ion, at m/z 107 in Fig. 2, shows that this ion can be loss scan set to monitor 17 u. An equivalent constant formed directly from the protonated molecule + neutral-loss spectrum shows a similar loss of NH3 from [M + H] or from the ammoniated species. This frag- the ammoniated chrysanthemic acid fragment ion, m/z mentation has been attributed to the formation of a 186 (see below). 1-ethynyl-2-methylpent-2-enyl cation (m/z 107), with Inspection of the electrospray spectrum of empen- the expulsion of a molecule of chrysanthemic acid thrin at a higher cone voltage (30 V) (Fig. 2) shows key (Scheme 3) as a result of heterolytic cleavage between ions at m/z 107 (base peak), 123, 151 and 169. An the chrysanthemic acid ester oxygen and the α-carbon electrospray product-ion spectrum (not shown), + of the 1-ethynyl-2-methylpent-2-enyl ester chain. The recorded following low-energy CID of the [M + H] constant neutral-loss scan in Fig. 3 also shows a ion (m/z 275) of empenthrin, reflects the connectivity significant loss of 28 u from m/z 107. It is proposed that between this ion and significant ions at m/z 107, 123, this loss involves the expulsion of ethene to yield the 151 and 169, while a further product-ion spectrum (not + m/z 79 ion, [C6H7] (Scheme 3). shown), recorded following low-energy CID of the m/z Further evidence related to the fragmentation of the 169 species, indicates connectivity between this ion and acid component of empenthrin, and subsequent forma- significant ions at m/z 69, 123 and 151. The ion at m/z tion of the protonated and ammoniated chrysanthemic 169 is attributed to the direct formation of protonated acids, was obtained in a parallel experiment using chrysanthemic acid, [C H O + H]+ , from the proto- 10 16 2 chrysanthemum monocarboxylic acid (Scheme 1). The nated molecule, following heterolytic cleavage between electrospray spectrum of this acid (not shown), the chrysanthemic acid ester oxygen and the α-carbon of the ethynylpentenyl chain accompanied by hydrogen recorded in the presence of CH3COONH4 and HCOOH and at a sampling cone voltage of 15 V, yields transfer (Scheme 2, pathway 1). + + The product-ion spectra of both the protonated the expected [M + H] and [M + NH4] ions at m/z molecule (m/z 275) and protonated chrysanthemic acid 169 and 186, respectively, in addition to the chrysanthe- fragment (m/z 169) from empenthrin indicate possible myl cation (m/z 123) and the chrysanthemyl acylium pathways for the formation of the chrysanthemyl ion (m/z 151). cation, m/z 123, which is observed in the spectrum A positive-ion electrospray spectrum of the propan- shown in Fig. 2. It is proposed that the chrysanthemyl 2-ol + H2O/CH3COONH4/HCOOH solvent in the cation can be formed directly from the protonated absence of analyte, at a cone voltage of 15 V, yields molecule following cleavage of the bond between the significant ions at m/z 61, 78 (base peak) and 121. The carbonyl carbon and cyclopropane ring (Scheme 2, ions at m/z 61 and 121 were assigned to the formation pathway 2) or indirectly from protonated chrysan- of protonated propan-2-ol and its protonated dimer, themic acid (Scheme 2, pathway 3). One route to the respectively. The base peak ion (m/z 78) in this fragment ion at m/z 151 (Figure 2) may involve the spectrum arises from adduction of propan-2-ol with + direct formation of the chrysanthemyl acylium ion from NH4 . At higher sampling cone voltages the intensities the protonated molecule following cleavage of the of all these ions are observed to diminish, with a carbon–oxygen bond α to the C = O group10 (Scheme corresponding increase in the population of less sig- 2, pathway 4). Another possible route to m/z 151 is via nificant, lower intensity ions. Minor ions observed at protonated chrysanthemic acid (Scheme 2, pathway 5). m/z 391, 279 and 149 are attributed to the fragmenta- Evidence that the chrysanthemyl acylium ion (m/z 151) tion of protonated bis(2-ethylhexyl)phthalate and asso-

Figure 2. Positive-ion electrospray spectrum of empenthrin (cone voltage 30 V).

© 1997 by John Wiley & Sons, Ltd. Rapid Communications in Mass Spectrometry, Vol. 11, 796–802 (1997) ELECTROSPRAY MASS SPECTROMETRY OF CHRYSANTHEMIC ACID PYRETHROIDS 799 ciated alkyl side-chain losses to form protonated cleavage and hydrogen transfer, appears in the electro- phthalic anhydride (m/z 149). spray spectrum as the protonated species. The electron ionization spectrum of empenthrin (Fig. Additional evidence that the major fragmentations 4) shows a dominant base peak ion at m/z 123 and a of empenthrin are associated with the carbonyl func- molecular ion of very low relative abundance ( < 0.5%), tion is given by an electron ionization product-ion which reflects the fissile nature of the bond between the spectrum of the molecular ion at m/z 274 (Fig. 5) which chrysanthemic acid ester oxygen and α-carbon of the yields key fragment ions at m/z 107, 123, 151 and 168. alcohol portion of the insecticide. The fragment ions at The spectra of empenthrin obtained under electrospray m/z 79, 91, 107, 123 (base peak) and 151 in this and electron ionization conditions contain a number of spectrum are also observed in the corresponding common fragment ions, which provides evidence that electrospray spectra of empenthrin whereas the m/z these species have similar internal energies. 168 ion (Fig. 4), which has been attributed to the The positive-ion ammonia CI spectrum of empen- formation of chrysanthemic acid following homolytic thrin (not shown) yields a base peak ion at m/z 107 + + together with the predicted [M + H] and [M + NH4] species and other significant ions which are identical with fragment ions observed in its electrospray spectrum.

Prallethrin (Mr = 300) The positive-ion electrospray spectrum of prallethrin, recorded at a lower cone voltage, (Fig. 6) demonstrates similar behaviour to empenthrin, yielding predom- + inantly the [M + NH4] ion base peak at m/z 318) and a signiﬁcant [M + H]+ ion at m/z 301. The electrospray spectra of prallethrin also show the same trends in fragment ion production at progressively higher cone voltages as were found for empenthrin, with a diminu- + tion of the [M + NH4] population, and a corresponding increase in the abundance of the [M + H]+ ion and of diagnostically useful fragment ions at m/z 123, 133, 151 and 169. All of these ions, with the exception of the m/z 133 species, are associated with the acid component of prallethrin and are observed to undergo similar fragmentations to those associated with the acid portion of empenthrin. The m/z 133 ion is attributed to the formation of a 2-methyl-4-oxo-3-prop-2-ynylcyclopent- 2-enyl cation, by expulsion of a molecule of chrysanthemic acid directly from the protonated molecule, [M + H]+ , following heterolytic cleavage between the chrysanthemic acid ester oxygen and the cyclopentenyl ring (Scheme 4). A precursor-ion scan (not shown) of the m/z 133 ion Scheme 2. Proposed major fragmentation pathways of empenthrin of prallethrin shows connectivity between this ion and following positive-ion electrospray ionization. the [M + H]+ ion and provides conﬁrmatory evidence

Figure 3. Empenthrin, positive-ion electrospray constant neutral-loss scan of 28 u.

for fragmentation, via m/z 301, to form the cyclopentenyl cation (Scheme 4). A positive-ion electrospray constant neutral-loss scan, recording a loss of 28 u (not shown), shows that the two significant ions to lose 28 u are m/z 151 and 133. The m/z 151 (chrysanthemyl acylium) ion loses carbon monoxide to form the chrysanthemyl cation, m/z 123. It is proposed that the m/z 133 ion also loses a molecule of carbon monoxide + to yield the m/z 105 ion, [C8H9] (Scheme 4). The appearance of significant ions ( < 10% relative abundance) in the electrospray spectra of empenthrin at m/z 297 and prallethrin at m/z 323, as the source cone voltage is increased, is attributed to the formation of sodiated adducts, [M + Na]+ . The ubiquitous nature of sodium makes it difficult to eliminate from analytical procedures. The increase in the relative abundance of the [M + Na]+ ion, at higher cone voltages, may be attributed to a greater stability of this species relative to + + the [M + H] and [M + NH4] ions. The positive-ion (ammonia) chemical ionization spectrum of prallethrin (not shown) yields the pre- + + dicted [M + H] and [M + NH4] (base peak) species Scheme 3. Formation of a 1-ethynyl-2-methylpent-2-enyl cation and as well as other significant ions which are also observed subsequent loss of a molecule of ethylene following positive-ion in its electrospray spectrum. In contrast to empenthrin, electrospray ionization of prallethrin. however, prallethrin does not yield a significant

Figure 4. Electron ionization spectrum of empenthrin.

Figure 5. Electron ionization product-ion spectrum following low-energy CID of the molecular ion of empenthrin (m/z 274).

© 1997 by John Wiley & Sons, Ltd. Rapid Communications in Mass Spectrometry, Vol. 11, 796–802 (1997) ELECTROSPRAY MASS SPECTROMETRY OF CHRYSANTHEMIC ACID PYRETHROIDS 801 chrysanthemyl acylium fragment ion at m/z 151 under The fragment ion observed at m/z 153 in the electron CI conditions. The electron ionization spectrum of ionization spectra of both insecticides (Figs 4 and 7) is prallethrin (Fig. 7) also shows the absence of a attributed to the loss of a methyl radical from the +· chrysanthemyl acylium ion (m/z 151), compared with chrysanthemic acid fragment ion, [C10H16O2] . Addi- spectra obtained under electrospray conditions. tional evidence of this loss is provided by an electron The electron ionization spectrum of prallethrin (Fig. ionization product-ion spectrum of prallethrin (not 7) shows the predicted chrysanthemyl cation base peak shown), following low-energy CID of the m/z 168 ion, +· +· (m/z 123) and a molecular ion [M] of low relative [C10H16O2] , which shows connectivity between this abundance ( < 4%). The ions at m/z 133 and 134 are ion and the m/z 153 ion. 11 equivalent to those reported by Crombie et al. for the An electron ionization product-ion spectrum, natural pyrethrin esters. These ions arise from hetero- recorded following low-energy CID of the chrysanthe- lytic cleavage between the chrysanthemic acid ester myl cation (m/z 123) from prallethrin indicates con- oxygen and the cyclopentenyl ring to form the cyclo- nectivity between this ion and significant ions at m/z 67, + pentenyl cation species, m/z 133 [C9H9O] and 134 + +· 69 and 81. It is proposed that the ion at m/z 69 [C5H9] [C9H10O] . It is noticeable that there is no significant is formed by elimination of a molecule of but-1,3-diene ion at m/z 134 in the electrospray spectrum of + from the chrysanthemyl cation [C9H15] . The ions at prallethrin, suggesting that formation of this odd- + + m/z 68 [C5H7] and 81 [C6H9] are consistent with the electron species is not a favoured fragmentation route. elimination of molecules of butene and propene, respectively, from the chrysanthemyl cation. The elimination of butene and propene under electron ionization conditions is in good agreement with the findings of Pattenden et al.,12 who proposed these losses while studying the mass spectra of the constituents of natural pyrethrum. These low-mass fragment ions are also observed in the electron ionization spectrum of empenthrin (Fig. 4) and in the electrospray spectra of both prallethrin and empenthrin (Scheme 2, pathway 7) at higher sampling cone voltages. Under electron ionization conditions, the chrysanthemyl cation (m/z 123) is common to a family of chrysanthemic acid ester insecticides. It appears, almost invariably as the base peak, in the spectra of the natural pyrethrins (cinerin I, jasmolin I, pyrethrin I) and of the synthetic pyrethroids (allethrin, cyphenothrin, prothrin, phenothrin and resmethrin) and represents an excellent class-specific ion for monitoring purposes. While electrospray mass spectrometry of chrysanthemic acid ester pyrethroids yields interpretable spectra at higher cone voltages, the increase in the population of low-mass fragment ions, as the source cone voltage is increased, makes the monitoring of class-specific ions less attrac- Scheme 4. Formation of a cyclopentenyl cation from heterolytic cleavage between the chrysanthemic acid ester oxygen and the tive (poor sensitivity) in electrospray mass spectrome- cyclopentenyl ring of prallethrin following positive-ion electrospray try than the monitoring of the respective protonated ionization. and/or ammoniated molecules.

Figure 6. Positive-ion electrospray spectrum of prallethrin (cone voltage 15 V). * Solvent ions.

Figure 7. Electron ionization spectrum of prallethrin.

CONCLUSION than the monitoring of their respective protonated and/ or ammoniated molecules. The positive-ion electrospray spectra of the chrysanthemic acid ester pyrethroids empenthrin and pralleth- Acknowledgements rin under low sampling cone voltage settings yield predominantly ammoniated molecule ions, and sig- The authors thank Dr Nobuo Ohno of Sumitomo Chemical for the nificant protonated molecule ions, thus showing that donation of research-grade samples and technical help provided. They also thank Professor Simon Gaskell and colleagues at the fragmentation via thermal degradation is not a major Michael Barber Centre for Mass Spectrometry, UMIST, for their help process. This contrasts with their positive-ion electron and support. ionization spectra, which show significant fragmentation associated with the fissile nature of the bond REFERENCES between the chrysanthemic acid ester oxygen and 1. F. B. LaForge, N. Green and M. S. Schechter, J. Org. Chem. 21, α-carbon/ring of the alcohol component of the mole- 455 (1956) cule and, as a result, yield molecular ions of low 2. J. P. Leahey (Ed.), The Pyrethroid Insecticides. Taylor and Francis, abundance. London (1985). The positive-ion (ammonia) CI spectra of prallethrin 3. T. Udagawa, S. Numata, K. Oda, S. Shiraishi, K. Kodaka and K. Nakatani, in Recent Advances in the Chemistry of Insect Control, and empenthrin contain the predicted ammoniated and Special Publication No. 53, edited by N. F. Janes, p. 192. Royal protonated molecules as well as other significant ions Society of Chemistry, London (1985) which are also observed in their electrospray spectra as 4. M. J. Bushell and R. Salmon, in Advances in the Chemistry of the sampling cone voltage is increased. In contrast to Insect Control III, Special Publication No. 147, edited by G. G. Briggs, p. 103. Royal Society of Chemistry, London (1994). empenthrin, however, prallethrin does not yield a 5. N. Matsuo, K. Tsushima, T. Takagaki, M. Hirano and N. Ohno, in significant chrysanthemyl acylium fragment ion under Advances in the Chemistry of Insect Control III, Special Publica- either electron or chemical ionization conditions. tion No. 147, edited by G. G. Briggs, p. 208. Royal Society of This study has demonstrated the capability of pos- Chemistry, London (1994). itive-ion electrospray mass spectrometry to generate 6. J. E. Casida (Ed.), Pyrethrum — The Natural Insecticide. Aca- demic Press, New York (1973). interpretable fragmentation spectra of chrysanthemic 7. M. Dole, L. Marck, R. Hines, R. Mobley and L. Ferguson, J. acid ester pyrethroid insecticides by source derived Chem. Phys. 49, 2240 (1968). collision-induced dissociations, and to yield diagnos- 8. J. B. Fenn, M. Mann, C. K. Meng, S. F. Wong and C. M. tically useful fragment ions that are identical with Whitehouse, Mass Spectrom. Rev. 9, 37 (1990). 9. D. Volmer, J. G. Wilkes and K. Levsen, Rapid Commun. Mass fragment ions observed in either their electron ioniza- Spectrom. 9, 767 (1995). tion and/or positive-ion (ammonia) CI spectra. 10. I. A. Fleet, J. J. Monaghan, D. B. Gordon and G. A. Lord, Analyst Whereas electrospray mass spectrometry of chrysan- 121, 55 (1996). themic acid ester pyrethroids yields interpretable spec- 11. L. Crombie, G. Pattenden and D. J. Simmonds, Pestic. Sci. 7, 225 (1976). tra at higher cone voltages, the associated increase in 12. G. Pattenden, L. Crombie and P. Hemesley, Org. Mass Spectrom. the population of low-mass fragment ions as the source 7, 719 (1973). sampling cone voltage is increased makes the monitoring of class-specific ions less attractive (poor sensitivity)